A typical swash plate type compressor as disclosed in, for example, U.S. Pat. No. 3,801,227 of Nakayama and U.S. Pat. No. 3,955,899 of Nakayama et al, having a pair of horizontal axially aligned cylinder blocks which form a combined block. Inside the combined block are formed axially extending cylinder bores, and the cylinder block is closed at both ends by front and rear housings, via valve plates. Centrally passing through the combined block, a drive shaft is rotatably supported by a suitable bearing means. To the middle of the drive shaft is fixed a swash plate operatively connected to, via ball bearings and shoes, double acting pistons slidably fitted in the cylinder bores. Thus, the rotating motion of the swash plate causes the reciprocal compressing motion of the pistons within the cylinder bores. The front and rear housings have refrigerant suction chambers and refrigerant discharge chambers, which are interconnected with the cylinder bores and are connectable to an outside air conditioning system by means of appropriate refrigerant flow pipelines. The above-mentioned respective shoes are normally shaped as a round plate having, at one side thereof, a flattened face slidably engaging with the swash plate. At the other side of the round plate, each shoe has a spherical recess for receiving therein the ball bearing. That is, the spherical recess of each shoe is shaped so as to be complementary with a part of the ball bearing, so that the entire surface of the spherical recess contacts a partial spherical surface of the ball bearing.
With the above-mentioned typical swash plate type compressor, when the compressor is used for compressing a refrigerant of the air conditioning system of a vehicle, the reciprocal pistons are susceptible to change in the speed of the compressing motion, since the compressor is driven by a drive force from the engine of the vehicle. Therefore, it often occurs that each piston of the compressor must assume an excess load in the case where the rotating speed of the engine of the vehicle is suddenly increased. Each piston of the compressor must also assume an excess load in the case where the compression of a liquid state refigerant must be carried out by the compressor due to unusual occurence of the direct introduction of the liquid state refrigerant into the compressor. On the other hand, during the ordinary and steady running state of the compressor, the load applied to the pistons thereof, as well as the shoes, is kept low. Therefore, pressure applied from the swash plate to the shoes is also kept low and distributed evenly over the entire surface of the flattened face of each shoe.
To the contrary, when each piston of the compressor must assume the above-mentioned excess load, the distribution of pressure applied from the swash plate to each shoe becomes quite uneven, in comparison with the case of the ordinary running state of the compressor. That is, the pressure is distributed in such a manner that a relatively high pressure is concentrated on a given central portion of each round shoe formed with the spherical recess. As a result, such central portion of each shoe must be subjected to an abrasion more rapidly than the other portions of each shoe surrounding the central portion. This rapid abrasion is caused by the frictional contact between the central portion and the swash plate under high pressure. In an extreme case, the frictional contact generates a high temperature friction heat causing the seizure of the shoe or shoes, whereby the compressor will finally be broken. The reason why the high pressure is concentratedly distributed in the central portion of each shoe is because when pressure is applied to each shoe from the swash plate, the central portion of each shoe being thinner than the other portion of the shoe is not able prevent a sufficient amount of resilient deformation occuring. Further, the central portion having a thinner thickness is easily affected by a temperature rise caused by the above-mentioned frictional heat. This fact results in the occurrence of the above-mentioned seizure of the shoe or shoes.